1. Field of the Invention
The present invention relates to an antenna for a backscatter-based RFID transponder (radio frequency identification) and a backscatter-based RFID transponder with an antenna of this type.
2. Description of the Background Art
The invention falls within the field of wireless and contactless communication. It falls in particular within the field of radio-based communication for the purpose of identifying articles, animals, persons, etc., and the transponders and remote sensors used for this purpose.
Although it can be used in principle in any contactless communication systems, the present invention and the problem on which it is based are explained below with reference to RFID communication systems and their applications. Here, RFID stands for radio frequency identification.
In RFID systems, data are transmitted bidirectionally with the use of high-frequency radio signals between a stationary or mobile base station, which is also often called a reading device, reader, or read/write device, and one or more transponders, which are attached to the articles, animals, or persons to be identified.
The transponder, which is also called a tag or label, typically has an antenna for receiving the radio signal emitted by the base station and an integrated circuit (IC) connected to the antenna. The integrated circuit in this regard comprises a receiving circuit for receiving and demodulating the radio signal and for detecting and processing the transmitted data. In addition, the integrated circuit has a memory for storing the data necessary for the identification of the appropriate article. Furthermore, the transponder may comprise a sensor, e.g., for temperature measurement, which, e.g., is also part of the integrated circuit. Such transponders are also called remote sensors.
RFID transponders may be used advantageously wherever automatic labeling, identification, interrogation, or monitoring is to occur. Articles such as, e.g., containers, pallets, vehicles, machines, luggage, but also animals or persons can be labeled individually with such transponders and identified without contact and without a line-of-sight connection. In the case of remotes sensors, in addition, physical properties or sizes can be determined and queried.
In the field of logistics, containers, palettes, and the like can be identified to determine the actual whereabouts, for example, during their transport. In the case of remote sensors, e.g., the temperature of the transported products or goods can be routinely measured and stored and read at a later time. In the field of protection from piracy, articles, such as, e.g., integrated circuits, can be provided with a transponder in order to protect unauthorized copies. In the commercial sector, RFID transponders can in many cases replace the bar code applied to products. There are additional applications, e.g., in the field of motor vehicles in antitheft devices or systems for monitoring air pressure in tires and in systems for access control for people.
Passive transponders do not have their own energy supply and obtain the energy necessary for their operation from the electromagnetic field emitted by the base station. Semi-passive transponders do have their own energy supply, but do not use the energy provided by it to transmit/receive data but, for example, to operate a sensor.
RFID systems with passive and/or semi-passive transponders, whose maximum distance from the base station is considerably greater than a meter, are operated within frequency ranges which are especially in the UHF or microwave range.
In such passive/semi-passive RFID systems with a relatively broad range, a backscattering method (backscattering) is generally used for data transmission from a transponder to a base station, during which a portion of the energy arriving at the transponder from the base station is reflected (backscattered). In this case, the carrier signal emitted by the base station is modulated in the integrated circuit of the transponder according to the data to be transmitted to the base station and reflected by means of the transponder antenna. Such transponders are called backscatter-based transponders.
In order to achieve the greatest range possible in backscatter-based transponders, it is necessary to supply as high a proportion as possible of the energy arriving from the base station at the transponder to the integrated receiving circuit of the transponder. Power losses of any type are to be minimized in this case. For this purpose, on the one hand, transponder antennas with a relatively broad receiving frequency range are required. Such relatively broadband antennas, in addition, can offer the advantage of fulfilling the requirements of several national or regional regulatory agencies with only one type of antenna. On the other hand, the energy picked up from the transponder antenna is to be supplied as undiminished as possible to the integrated receiving circuit, which typically has a capacitive input impedance, i.e., an impedance with a negative imaginary part.
German Patent Application DE 103 93 263 T5 discloses an antenna for an RFID system, which has a planar spiral structure with two arms. Proceeding from a central area, the two arms extend each in a spiral outwardly in a full turn. The input impedance of this antenna is also capacitive.
A disadvantage here is that the impedance of this antenna deviates greatly from the conjugate complex value of the impedance of the chip input circuit and, for this reason, an additional, separate matching circuit with a coil and a capacitor is required between the antenna and chip. Because of parasitic resistances in these elements, there are power losses on the transponder side, which reduce the range in a deleterious way. Furthermore, the separate matching circuit limits the freedom in the placement of the chip and causes more complicated and therefore more cost-intensive implementations of the transponder.